The term “conversion coating,” as used in the metal finishing industry, refers to the conversion of a metal's surface into a surface that will more easily accept applied coatings and/or provide for a more corrosion resistant surface. These coatings are typically thin (not more than 600 nm thick on aluminum), quickly and easily formed, and, if used to enhance paint adhesion, conversion-coated shortly before painting to prevent intermediate degradation of the conversion coating.
Conversion coatings for aluminum have been in use since the early 1920s, and there are a number of different products on the market. Hexavalent chromium-based conversion coating systems have been used for more than 60 years because they have provided superior corrosion resistance and paint adhesion characteristics when used with aluminum and its alloys. Electrolytically generated, or anodized aluminum oxide surfaces have long been used to give the highest quality and range of aluminum oxide-based conversion coatings. However, this quality comes at a cost in terms of time, money and effort.
In most metal processing procedures (painting, conversion coating, anodizing, etc.), an important step is the proper cleaning of the metal surfaces prior to processing. Cleaning removes surface oils and loose dirt from the substrate surface. In general, alkaline cleaners have been used to provide superior results.
Deoxidation refers to the removal of oxides and other inorganics, that would otherwise interfere with further processing of the aluminum, without significant attack upon the aluminum surface. To prevent excessive attack, deoxidizers generally contain an oxidizing agent designed to maintain a thin film of oxide on the metal's surface. This allows for the oxide to be removed, rather than having a direct attack on the metal substrate by the deoxidizer.
The best deoxidizers for aluminum are those based on nitric acid coupled with another oxidizer, such as, for example, hydrogen peroxide or sodium bromate, etc. Unlike other acids, nitric acid will dissolve aluminum oxide but has very little effect upon aluminum itself. Chromic acid and/or chromates have been used in deoxidizers in conjunction with nitric acid and are generally preferred. However, in addition to the toxicity issue associated with the use of hexavalent chromium, such chromium-based deoxidizers leave a thin deposit of chromium oxides on the metal's surface.
Most commonly used conversion coats contain chromium, fluorine, and often contain proprietary additives. The conversion coats operate at a pH ranging from about 1-2. When the freshly deoxidized and rinsed aluminum substrate is immersed in the conversion coat, there is an acidic attack at the interface of solution and substrate. This raises the pH of the solution at the interface, allowing chromium to precipitate. The conversion coating consists of layers of increasingly rich chromium compounds. The treated metal usually will have a clear to light gold, iridescent finish. The coating is quite hard and scratch resistant, and withstands temperatures up to the melting point of the aluminum, and will not degrade over time.
Hexavalent chromium-based conversion coating systems have been used widely with aluminum (and its alloys) because they have provided superior corrosion resistance and paint adhesion characteristics when used with aluminum and its alloys. However, the discovered toxicity of hexavalent chromium has imposed additional regulatory concerns now requiring additional safety and waste treatment procedures for processing. In addition, the handling, storage and disposal of hexavalent chromium waste increases the overall cost for conversion coating processing lines.